Amounts of traffic on mobile communication networks have sharply increased in recent years due to the proliferation of smartphones. Ratios of downlink (DL) traffic and uplink (UP) traffic fluctuate depending on places and/or time slots. For this reason, it is desirable to efficiently use radio resources by flexibly adjusting an amount of radio resources for DL and an amount of radio resources for UL according to an amount of traffic.
Long Term Evolution (LTE) supports Frequency Division Duplex (FDD) and Time Division Duplex (TDD) as duplex communication systems. In TDD, a ratio of UL subframes and DL subframes in a radio frame can be changed by dynamically changing UL/DL configurations according to an amount of traffic. In other words, in TDD, an amount of radio resources for DL and an amount of radio resources for UL can be flexibly adjusted. On the other hand, since a DL band and a UL band are pre-determined in FDD, it is not possible to flexibly adjust an amount of radio resources for DL and an amount of radio resource for UL. Although it is also possible in FDD to use more DL bands and fewer UL bands through carrier aggregation, as a result, a use rate of radio resources of UL bands decreases, and thus it is hard to say that radio resources are efficiently used.
Thus, a technique of using a UL band of FDD for both DL and UL in a time division manner has been discussed. The technique can be called flexible duplex. Patent Literature 1, for example, discloses a technology of using UL bands of FDD in a TDD mode.